Rock bolts are long anchor bolts used to stabilise excavations in rock, such as tunnels and rock faces. A rock bolt transfers load at the exterior surface of the rock into the interior mass of the rock. Anchor bolts are used to securely attach objects to rock or concrete surfaces.
The 1890s first saw the use of rock bolts. The St Joseph Lead Mine in the USA in the 1920s is recorded as having used rock bolts.
Australia and the USA have both been recorded as using rock bolts in civil applications in the late 1940s. In 1947 Australian engineers were reported as experimenting with four meter long expanding anchor rock bolts during work on the Snowy Mountain scheme.
Rock bolts are typically installed in a pattern, the actual arrangement depending on the type of rock (rock quality—position and type of fractures already present, strength of the rock and its propensity to fracture etc.), the type of excavation (tunnel, cut face etc.) and the surrounding geology/geography (risk of seismic activity and any nearby underground or overground workings/structures).
Both rock bolts and anchor bolts can be used to retain a metal (wire) mesh over a rock face to reduce risk of loose material or rock fall that might injure personnel, damage vehicles/equipment and/or block a tunnel.
As with anchor bolts, there are many types of proprietary rock bolt designs. Typically a mechanical means, epoxy means or combination of both is used to set the bolt into the rock/concrete.
Rock bolts work by ‘knitting’ the rock mass together sufficiently before it can move enough to loosen and fail. Rock bolts can become ‘seized’ throughout their length by small shears in the rock mass, so they are not fully dependent on their pull-out strength.
In the case of a rock bolt, it is important to ensure that the rock bolt is capable of retaining the rock in situ when installed. In the case of an anchor bolt, it is important to ensure the item secured by the bolt is safely retained.
Static testing is an alternative form of test. This can be carried out in a laboratory or in situ. A continuous load is applied to the rock bolt, usually hydraulically. However, static testing does not simulate the ‘shock’ loading to the bolt present in dynamic testing.
Dynamic tests are conducted to ensure the respective bolt can operate as required. For rock bolts, a dynamic test is carried out in laboratory using a simulated bore-hole whereby the rock bolt is secured in a cement/resin mix inserted into a hollow (steel) tube. The tube is supported as a load acts on the head of the rock bot. This involves hydraulically applying a pull out force to the rock bolt.
Whilst laboratory simulation is useful, it does not accurately recreate working conditions and cannot perform an in-situ dynamic test on a bolt for the actual rock. Laboratory dynamic testing involves setting the rock bolt in the tube and suspending the tube and rock bolt from a raised support. A weight is dropped a preset distance to apply a shock load to the head of the bolt. The amount of weight and distance dropped determines the amount of force applied to the rock bolt.
Another form of laboratory testing involves dropping the rock bolt and tube combination together with a weight attached to the rock bolt. Fall of the rock bolt and tube is arrested once the required velocity is reached, but the weight is allowed to continue and thereby applies a load to the rock bolt. This method is said to better simulate the movement of the rock before the rock face fails (i.e. during a seismic event). Such testing is carried out by the Western Australian School of Mines (WASM) and is known as the WASM momentum transfer concept.
With the aforementioned in mind, the resent invention has been developed in order to provide improved in situ dynamic testing for rock bolts (and optionally anchor bolts).